Tolvaptan represented by the following formula (2) is a known compound, and has been disclosed in, for example, U.S. Pat. No. 5,258,510 specification (Example 1199).

It is known that tolvaptan is useful as a vasopressin antagonist having aquaretic activity (Circulation, 107, pp. 2690-2696 (2003)). However, because of its low water solubility, tolvaptan has problems in that it is poorly absorbed by the intestinal canal, its dosage form and administration route are limited, etc. Although attempts have been made to solve these problems so that, for example, tolvaptan can be administered in the form of an amorphous solid preparation composition (Japanese Unexamined Patent Publication No. 1999-21241), in the application of tolvaptan, its dosage form and administration route still remain limited.